Nanocomposite material

ABSTRACT

The invention relates to a nanocomposite material on the basis of clay having a layered structure and a cation exchange capacity of from 30-250 milliequivalents per 100 gram, a polymeric matrix and a block copolymer or a graft copolymer, which block copolymer or graft copolymer comprises one or more first structural units (A), which are compatible with the clay and one or more second structural units (B), which are compatible with the polymeric matrix.

The invention relates to a nanocomposite material, to a process for thepreparation thereof and to a modified clay.

In the past decades, it has already often been proposed to improve theproperties of polymeric materials by including in these materials aspecific amount of a clay. The presence of a clay in a polymericmaterial particularly contributes to properties such as the mechanicalstrength and the heat resistance of the polymeric material.

A great difficulty when including a clay in a polymeric matrix residesin the mutually rather different nature of the materials. The polymericmaterial of the matrix is a usually non-polar organic material, whereasthe clay is a much more polar inorganic material. Because of thisdifference the materials are poorly intermixable; they are intrinsicallynot mixable.

To circumvent this difficulty, it has been proposed to carry out thesynthesis of the polymer that forms the matrix, a polymerizationreaction, in the presence of the inorganic material. The idea was thatthe clay is perhaps more easily intermixable with a monomeric materialthan with a polymeric material. This method, however, proved to resultin an inhomogeneous product which does not have the desired properties.The clay has a layered structure which cannot be broken easily, so thata homogeneous mixing is hard to obtain.

U.S. Pat. Nos. 4,889,885 and 4,810,734 propose to first swell the claybefore adding monomeric material and carrying out a polymerization. Tothis end, the clay is modified with a swelling agent which increases themutual distance between the layers of the clay structure to such anextent that monomers fit therebetween. After polymerization of themonomers polymeric material is then automatically located between theclay layers.

The swelling agent described in the above patents is so-called oniumions. Within this context an onium ion is a surfactant with a head groupformed by an ammonium, pyridinium, sulfonium or phosphonium group, andone or more non-polar tails. The cationic head group of an onium ion isexchanged for cations between the crystalline layers of the clay. Thetails must have a functional group capable of entering into a bondinginteraction with the monomeric material, so that the polymers are formedbetween the layers of the clay.

Nevertheless, swelling with an onium ion also often proved ineffectivein obtaining a proper mixing of the clay with a polymeric matrix.European patent application 0 747 322 describes that even when an oniumion with two non-polar tails is used, additional measures are necessaryto homogeneously disperse a clay in a polymeric matrix whichparticularly consists of rubbery materials. Accordingly, the aboveEuropean patent application proposes to introduce, in addition to anonium ion with two non-polar tails, one or more host molecules, alsosurfactant-like molecules, between the clay layers. A drawback of thismethod is that it is very complicated and does not in all polymericmaterials enable a homogeneous dispersion of clay in the polymericmatrix.

Another approach is described in U.S. Pat. No. 5,578,672. This approachcomprises two steps. In the first step, an aqueous layered clay isswollen with monomers, oligomers or polymers that are compatible withwater. This leads to a partly hydrophilic material. This first step isoften referred to as the so-called intercalation. The distance betweenthe clay layers is thereby increased. The second step comprises themixing of the intercalated clay with a second polymer. This step isreferred to as the so-called exfoliation and must lead to looseindividual clay plates. The resulting product is finally included in thedesired polymeric matrix through extrusion. In this procedure it isessential that the clay contains a specific minimum content (usually atleast 5%) of water. The method described in this U.S. patent is ratherlaborious and complicated. Moreover, this method is not applicable tomany polymeric matrices, so that a clay cannot be included in everypolymeric material.

The international patent application WO-A-93/04118 describes a compositematerial on the basis of a polymeric matrix and a clay, which clay ismodified with a specific compound. This specific compound consists of asilane or an onium group and a group compatible with the polymericmatrix. It has turned out that with this specific compound a substantialdispersion of a clay in a polymeric matrix is only obtainable with nylonas polymeric matrix.

It is an object of the invention to provide a nanocomposite material inwhich a clay is very homogeneously dispersed in a polymeric matrix,which nanocomposite material is easy to prepare. The invention thereforerelates to a nanocomposite material on the basis of a clay having alayered structure and a cation exchange capacity of from 30 to 250milliequivalents per 100 gram, a polymeric matrix and a block copolymeror graft copolymer, which block copolymer or graft copolymer comprisesone or more first structural units (A), which are compatible with theclay, and one or more second structural units (B), which are compatiblewith the polymeric matrix.

It has been found that by using a block copolymer or graft copolymer ofthe above-mentioned type a clay can be very homogeneously mixed with apolymeric matrix. Moreover, by suitably selecting the structural unitsof the block copolymer or the graft copolymer a clay can be included ina polymeric matrix of any desired nature. A nanocomposite materialaccording to the invention has very favorable properties, such as agreat heat resistance, a great mechanical strength, in particular agreat tensile strength and a good impact resistance, a low electricconductivity, a high glass transition temperature and a very lowpermeability to gases, such as oxygen or water vapor, and liquids, suchas water or solvents.

A nanocomposite material according to the invention is, as statedbefore, based on a clay having a layered structure. The clay may be of anatural or synthetic nature. Preferably, the clay has a large contactsurface.

Very suitable are clay types based on layered silicates, such as layeredphyllosilicate composed of magnesium and/or aluminum silicate layerswhich are each about 7-12 Å in thickness. Especially preferred aresmectite-like clay minerals, such as montmorillonite, saponite,hectorite, fluorohectorite, beidellite, nontronite, vermiculite,halloysite and stevensite. These materials impart very favorablemechanical properties and a great heat resistance to a nanocompositematerial.

A suitable clay type has a cation exchange capacity of from 30 to 250milliequivalents per 100 gram. When this capacity exceeds the aboveupper limit, it proves difficult to finely disperse the clay on amolecular level because of the strong mutual interaction of the claylayers. When the cation exchange capacity is lower than the above lowerlimit, it turns out that the clay is hard to modify, owing to the factthat the interaction with the block copolymer or graft copolymer issmall. There is preferably used a clay having a cation exchange capacityof from 50 to 200 milliequivalents per 100 gram.

The polymeric matrix present in a nanocomposite material according tothe invention can be formed by any polymeric material. Both homopolymersand copolymers may serve as polymeric matrix. It is one of theadvantages of the invention that by selecting the block copolymer or thegraft copolymer any polymeric matrix can be modified with a clay of theabove-described nature. Accordingly, by providing the rightconstituents, e.g. in the form of a kit, the skilled worker is affordedan opportunity to prepare for any desired use a combination of aspecific clay and a specific polymeric material, and thus a desirednanocomposite material.

Polymeric materials suitable as polymeric matrix in a nanocompositematerial according to the invention are both polyadducts andpolycondensates. Examples are polyolefins, such as polyethylene orpolypropylene, vinyl polymers, such as polystyrene or polymethylmethacrylate, polyesters, such as polyethylene terephthalate orpolycaprolactone, polycarbonates, polyaryl ethers, polysulfones,polysulfides, polyamides, polyetherimides, polyether esters, polyetherketones, polyether ester ketones, polyvinyl chloride, polyvinylidenechloride, polyvinylidene fluoride, polysiloxanes, polyurethanes andpolyepoxides. There are preferably used polyolefins, vinyl polymers,polyesters, polyethers, polysiloxanes or acrylic polymers, because theproperties of these materials show a substantial improvement due to thepresence of a clay.

Besides on the clay described before and the polymeric matrix, ananocomposite material according to the invention is based on a blockcopolymer or a graft copolymer. This block copolymer or graft copolymeris a polymer comprising first structural units (A), which are compatiblewith the clay, and one or more second structural units (B), which arecompatible with the polymeric matrix. When the structural units occur ina straight polymeric chain, reference is made to a block copolymer. Whenthe structural units (A) occur in a chain which is a branch of the chainin which the structural units (B) occur, or vice versa, reference ismade to a graft copolymer.

The structural units (A) are compatible with the clay. By this is meantthat these units in themselves, i.e. not in the copolymeric form withthe structural units (B), are excellently mixable with the clay. Thestructural units (A) are preferably of a hydrophilic nature. Materialssuitable as structural units (A) are polyvinylpyrrolidone, polyvinylalcohol, polyethylene oxide, linear or dendritic polyethylenimine,polyoxymethylene, polytetrahydrofuran, polyacrylic acid, polymethacrylicacid, polydimethylacrylamide, polymethylacrylamide, copolymers ofacrylic acid or methacrylic acid and acrylamide, polyisopropylaride,starch, polysaccharides and cellulose derivatives. It is preferred thatat least one of the structural units (A) is derived from monomeric unitsselected from the group of vinylpyrrolidone, vinyl alcohol, ethyleneoxide, ethylenimine, vinylpyridine, acrylic acid and acrylamide. Thesepreferred units (A) are excellently compatible with a clay.

Very suitable materials for use as structural units (A) have a molecularweight of from 100 to 5,000, preferably from 1,000 to 3,000. It is alsoadvantageous when the material of the structural units (A) contains from5 to 20 monomeric units.

The structural units (B) are compatible with the polymeric matrix. Bythis is meant that these units in themselves, i.e. not in thecopolymeric form with the structural units (A), are excellently mixablewith the material of the polymeric matrix. It is also possible that thenature of the structural units (B) is the same as the nature of thepolymeric matrix. An example is a polymeric matrix of polyethylenehaving a molecular weight of 5,000 and structural units (B) ofpolyethylene having a molecular weight of 2,500. It is even possiblethat the material of the polymeric matrix is exactly equal to that ofthe structural units (B). In the above example, the structural units (B)could then be of polyethylene having a molecular weight of 5,000.

The nature of the structural units (B) will depend on the nature of thepolymeric matrix. Materials suitable as structural units (B) are, e.g.,polyolefins, such as polyethylene or polypropylene, vinyl polymers, suchas polystyrene or polymethyl methacrylate, polyesters, such aspolyethylene terephthalate or polycaprolactone, polycarbonates, polyarylethers, polysulfones, polysulfides, polyamides, polyetherimides,polyether esters, polyether ketones, polyether ester ketones, polyvinylchloride, polyvinylidene chloride, polyvinylidene fluoride,polysiloxanes, polyurethanes and polyepoxides. There are preferably usedpolyolefins, vinyl polymers, polyesters, polyethers, polysiloxanes oracrylic polymers.

According to a preferred embodiment, a block copolymer or a graftcopolymer is used in which the structural units (A) contain at least 2monomeric units and the structural units (B) contain the same or alarger amount of monomeric units as/than the structural units (A). Ithas been found that with such a block copolymer or graft copolymer avery finely divided homogeneous dispersion of the clay is obtained inthe polymeric matrix.

In a nanocomposite material according to the invention, the weight ratioof the amount of block copolymer or graft copolymer to the amount ofclay is preferably between 0.01:1 and 100:1, with a special preferencebetween 0.05:1 and 6:1. The weight ratio of the amount of clay to theamount of polymeric matrix is preferably between 1:200 and 2:1, with aspecial preference between 1:50 and 1.2:1.

The invention further relates to a process for preparing a nanocompositematerial as described above. It should be noted that it is possible inthis connection to first bring together the clay and the block copolymeror graft copolymer or first bring together the polymeric matrix and theblock copolymer or graft copolymer and only then add the required thirdconstituent. It is further possible to simultaneously bring together allthe three required constituents, namely clay, polymeric matrix and blockcopolymer or graft copolymer.

It is preferred, however, to first modify the clay with a blockcopolymer or graft copolymer of the above-described nature. Theinvention therefore also relates to a modified clay suitable forpreparing a nanocomposite material as described above, based on a clayhaving a layered structure and a cation exchange capacity of from 30 to250 milliequivalents per 100 gram, which clay is modified with a blockcopolymer or a graft copolymer, which block copolymer or graft copolymercomprises one or more first structural units (A), which are compatiblewith the clay, and one or more second structural units (B). Thismodified clay can then be suitably mixed with a polymeric matrix. Bysuitably selecting the structural units (B), a skilled worker is capableof including a clay in a polymeric matrix of any desired nature.

When preparing a nanocomposite material according to the invention, inany of the above-mentioned sequences of bringing together, it ispreferred to grind or pulverize the clay previously. Such a pretreatmentof the clay results in an easier and better mixability of the differentconstituents.

The constituents of a nanocomposite material according to the inventionmay be brought together in any suitable manner, provided this mannergives a good mixture. Examples of methods of bringing together theconstituents comprise agitation for a longer period of time at elevatedtemperature and extrusion. Suitable mixing conditions depend on thenature of the selected constituents and can be easily determined by askilled worker. The agitation may be carried out, e.g., at a temperaturebetween 40 and 80° C. and the extrusion, e.g., between 40 and 150° C. ina twin-screw extruder.

The nanocomposite materials according to the invention may be verysuitably used for a great diversity of applications. The materials areexcellently processable and can be molded in conventional molding steps,such as injection molding and extrusion processes. Molded articles ofdifferent nature can be prepared from the present nanocompositematerial. Examples comprise any application for which the material ofthe polymeric matrix is suitable. As preferred applications, packagingand construction materials may be mentioned.

The invention will now be explained in more detail with reference to thefollowing examples.

EXAMPLE I

A smectic clay mineral (montmorillonite, 1 g) having a cation exchangecapacity of 85 milliequivalents per 100 gram was mixed together with 1.3g of a block copolymer consisting of one polyethylene oxide block (PEO)and one polystyrene block (PS) for 3 hours by agitating at a temperatureof 80° C. The molecular weight of the PEO block was about 1,000 and thatof the PS block about 3,000.

The resulting material was characterized with X-ray diffraction anddifferential scanning calorimetry. This characterization showed thatsubstantial exfoliation had occurred.

Subsequently, the exfoliate was extruded together with a styrenehomopolymer. The final product had a clay content of 5% by weight, basedon the final product. from the results of studies by means of X-raydiffraction and electron microscopy it was determined that the clay washomogeneously dispersed in the styrene homopolymer.

The tensile force modulus, determined according to DIN 53455, of themodified styrene homopolymer was compared with that of the non-modifiedstyrene homopolymer, which showed that the tensile force modulus of themodified material was 10% higher.

EXAMPLE II

A smectic clay mineral (bentonite, 1 g) having a cation exchangecapacity of 85 milliequivalents per 100 gram was suspended at 50° C intetrahydrofuran and agitated for 3 hours with 1.3 g of a block copolymerconsisting of one poly-4-vinylpyridine block (P4VP) and one polystyreneblock (PS). The molecular weight of the P4VP block was about 3,000 andthat of the PS block about 27,000.

The resulting material was characterized with X-ray diffraction anddifferential scanning calorimetry. This characterization showed thatpartial exfoliation had occurred.

The exfoliated material was extruded together with a styrenehomopolymer. There was thus obtained a final product which containedhomogeneously dispersed, fully exfoliated clay plates. The clay contentof the final product was 50% by weight, based on the final product.

EXAMPLE III

A smectic synthetic clay mineral (saponite, 1 g) having a cationexchange capacity of 83 milliequivalents per 100 gram was suspended at50° C. in tetrahydrofuran and agitated for 3 hours with 1.3 g of a blockcopolymer consisting of one dendritic polyethylenimine block (dend-P₈PEI) and one polystyrene block (PS). The molecular weight of the dend-P₈PEI block was about 1,000 and that of the PS block about 2,000.

By means of X-ray diffraction it was determined that the layered mineralstructure was intercalated to a structure having a mutual distancebetween the layers of 12.7 Å.

Coextrusion with a styrene homopolymer led to a clear transparentmaterial with exfoliated clay layers. The clay content of the finalproduct was 5% by weight, based on the final product.

EXAMPLE IV

A montmorillonite (1 g) having a cation exchange capacity of 105milliequivalents per 100 gram was suspended in tetrahydrofuran and mixedfor 3 hours with 1.3 g of a multiblock copolymer consisting of onedendritic polyethylenimine core block (dend₁₆) functionalized with 16octadecyl groups (block B, PE-compatible). The molecular weight of thedendritic polyethylenimine core block was 1,600.

An X-ray diffraction study of the resulting material showed that thelayered mineral structure was intercalated with the multiblockcopolymer. The mutual distance between the intercalated clay layers was30.4 Å.

The intercalated material was extruded together with polyethylene. Thefinal product contained 5% by weight of completely exfoliated clay,based on the final product, and was clear and transparent.

The tensile force modulus, determined according to DIN 53455, of themodified styrene homopolymer was compared with that of the non-modifiedstyrene homopolymer, which showed that the tensile force modulus of themodified material was 100% higher.

I claim:
 1. A process for preparing a nanocomposite material consistingessentially of a clay having a layered structure, a polymeric matrix anda block copolymer or a graft copolymer, said process comprisingexfoliating a clay having a layered structure and a cation exchangecapacity of from 30 to 250 milliequivalents per 100 gram with a blockcopolymer or a graft copolymer, which block copolymer or graft copolymercomprises one or more first structural units (A), which are compatiblewith the clay, and one or more second structural units (B), which arecompatible with the polymeric matrix, and mixing the block or graftcopolymer exfoliated clay with said polymeric matrix.
 2. The process ofclaim 1, wherein said exfoliating is carried out in the presence of saidpolymeric matrix.
 3. The nanocomposite material formed by the process ofclaim
 1. 4. The nanocomposite material of claim 3, wherein the weightratio of the amount of block copolymer or graft copolymer to the amountof day is between 0.05:1 and 6:1.
 5. The nanocomposite material of claim3 wherein the weight ratio of the amount of clay to the amount ofpolymeric matrix is between 1:50 and 1,2:1.
 6. A molded article madefrom a nanocomposite material prepared according to the process ofclaim
 1. 7. A nanocomposite material according to claim 3, wherein theclay has a cation exchange capacity of from 50 to 200 milliequivalentsper 100 gram.
 8. A nanocomposite material according to claim 3, whereinthe polymeric matrix is selected from the group consisting ofpolyolefins, vinyl polymers, polyesters, polyethers, polysiloxanes andacrylic polymers.
 9. A nanocomposite material according to claim 3,wherein the structural units (A) have a number average molecular weightof from 100 to 5,000 and the structural units (B) have a number averagemolecular weight of from 100 to 20,000.
 10. A nanocomposite materialaccording to claim 3, wherein the structural units (A) contain at least2 monomeric units, and wherein the structural units (B) contain the sameor a larger amount of monomeric units as/than the structural units (A).11. A nanocomposite material according to claim 10, wherein thestructural units (A) contain from 5 to 20 monomeric units.
 12. Ananocomposite material according to claim 3, wherein at least one of thestructural units (A) is derived from monomeric units selected from thegroup of vinylpyrrolidone, vinyl alcohol, ethylene oxide, ethyleneimine,vinylpyridine, acrylic acid and acrylamide.
 13. A nanocomposite materialaccording to claim 3, wherein the weight ratio of the amount of blockcopolymer or graft copolymer to the amount of clay is between 0.01:1 and100:1.
 14. A nanocomposite material according to claim 3, wherein theweight ratio of the amount of clay to the amount of polymeric matrix ispreferably between 1:200 and 2:1.
 15. A modified clay suitable forpreparing a nanocomposite material according to claim 3, based on a clayhaving a layered structure and a cation exchange capacity of from 30 to250 milliequivalents per 100 gram, which clay is exfoliated with a blockcopolymer or a graft copolymer, which block copolymer or graft copolymercomprises one or more first structural units (A), which are compatiblewith the clay, and one or more second structural units.
 16. Ananocomposite material prepared according to the process of claim 1,comprising a clay having a layered structure and a cation exchangecapacity of from 30 to 250 milliequivalents per 100 gram, a polymericmatrix and a block copolymer or a graft copolymer, which block copolymeror graft copolymer comprises one or more first structural units (A),which are compatible with the clay, and one or more second structuralunits (B), which are compatible with the polymeric matrix.
 17. A processfor preparing a nanocomposite material comprising a clay having alayered structure, a polymeric matrix and a block copolymer or a graftcopolymer, said process consisting essentially of exfoliating a clayhaving a layered structure and a cation exchange capacity of from 30 to250 milliequivalents per 100 gram with a block copolymer or a graftcopolymer, which block copolymer or graft copolymer comprises one ormore first structural units (A), which are compatible with the clay, andone or more second structural units (B), which are compatible with thepolymeric matrix, and mixing the block or graft copolymer exfoliatedclay with said polymeric matrix.
 18. The nanocomposite material formedby the process of claim
 17. 19. A nanocomposite material according toclaim 18, wherein the clay has a cation exchange capacity of from 50 to200 milliequivalents per 100 gram.
 20. A nanocomposite materialaccording to claim 18, wherein the structural units (A) have a numberaverage molecular weight of from 100 to 5,000 and the structural units(B) have a number average molecular weight of from 100 to 20,000.